An image may be encoded into image data, which may be transmitted, stored, processed, or otherwise manipulated electronically. The image may be decoded and converted to hard copy by sending the image data to a printer. Laser thermal printers, with their high resolution capability, provide a popular mode for producing hard copy images from digital image data.
Laser thermal printers may be used with a variety of imaging media or “receptors,” including many kinds of film and paper. In a typical laser imager, a receptor is placed very close to a color-coated substrate or “donor” sheet, and one or more laser beams are directed at the donor. Each laser may emit an infrared beam, and the colored coating, which may contain a colorant and/or an infrared-sensitive dye, heats when exposed to a beam of sufficient power. The resulting thermal energy induced by the lasers triggers the imaging process, causing colorant to transfer from the donor to the receptor.
The lasers are typically arranged in a linear array, with each laser in the array individually modulated by image data. The array may include any number of lasers, although an array of sixteen lasers is typical. Semiconductor or “diode” lasers are commonly used in an array for reasons of cost, convenience and reliability. The lasers may, for example, emit infrared beams with wavelengths of approximately 830 nm. The breadth of the array, which is a function of the spacing of the lasers, is usually adjustable.
The image data that modulate the lasers represent the shape, size and density of the image. Image data are routinely stored electronically, and are provided to the array in the form of a plurality of signals, typically one signal for each laser. Although the lasers in the array strike only a small portion of the donor and receptor at any one time, the array can print large regions by scanning across the donor and receptor. As the array scans the donor and receptor, each laser in the array emits a beam in response to an image signal. In most cases, the laser array may make several successive parallel or helical passes to generate the complete image. Each pass of the array prints a strip or “swath” on the receptor. To avoid the appearance of white lines in the receptor, i.e., unprinted spaces between swaths, successive swaths may abut or overlap preceding swaths.
When a beam sufficiently heats the donor, a spot of colorant is transferred from the donor to the receptor. By modulating the duration for which a laser beam strikes an area on the donor, modulating a laser's intensity and/or modulating the size of the beam, spots of colorant of different sizes may be formed, and thereby colors may appear darker or fainter in color. Often a region of the receptor is intended to receive no colorant from the donor, and when scanning that region, a laser emits no beam, or emits a beam at a power level below that needed to transfer colorant.
The receptor may be scanned multiple times using donors of different colorants, creating a multicolor image by the superposition of multiple monochromatic images. By repeating scans with donors coated with cyan, yellow, magenta and black, for example, a multicolor image may be formed on the receptor. For high fidelity printing systems, additional colors such as green and orange may be provided.
Ideally, an observer ought to be able to look at a printed image and see no indications that the image had been formed by repeated passes of a laser array. In some cases, however, unintended patterns, such as groups of lines or streaks, appear in the printed image. These imaging aberrations, also called “artifacts,” may be evident in halftone printing, when printing a single color and when overprinting multiple colors.
At least two factors contribute to these artifacts. One factor is the formation of “swath lines,” which may manifest at an edge of a laser swath in the form of heavier colorant depositions. Swath lines may result from the deposition of excess colorant along an edge of the swath, causing a heavier line of colorant than intended and causing the swath to have a non-uniform distribution. Swath lines may also result from a deficit of colorant. The end lasers in the array, i.e., the outermost lasers in the array, have one neighbor laser instead of two neighbor lasers. As a result, the lines on the edge of the scan may receive less thermal energy, causing less transfer of colorant.
Ruling and screen angle can also contribute to the artifacts. In halftone printing, printed images are formed from halftone dots, with the halftone dots varying in size according to the lightness or darkness of the image. The halftone dots are printed by the lasers, but the halftone dots are generally much bigger than the laser beams. Each laser prints in units of “pixels,” and usually a matrix of several pixels is required to make up a single halftone dot. Consequently, it may take several lasers in the array to print a single halftone dot. Halftone dots are printed at a defined ruling, i.e., a number of halftone dots per unit of length, and at a defined screen angle, i.e., an angle at which the rows of halftone dots are oriented.
In standard four-color printing, each color is printed using approximately the same ruling, but each color is “screened” at a different angle to prevent halftone dots of different colors from printing one on top of the other and obscuring each other. Screening colors at different angles also avoids generation of an objectionable moiré pattern between the color layers.
When the spatial frequency of the swath lines and the spatial frequency of the halftone dots are close to one another but not equal, a moiré pattern may form. In other words, two small superimposed patterns of similar spatial frequency may combine visually to form a larger and more prominent moiré pattern. The moiré pattern usually resembles spurious light and/or dark bands in the image, and is generally considered to be an undesirable aberration of the image. The phenomenon may also be described as “beating,” because it results from superposition of patterns close to each other in spatial frequency. In a multicolor image, a plurality of screen angles come into play, and beating may occur in one or more colors, resulting in superimposed moiré patterns.
Stochastic printing is an alternative to halftone printing, in which color density is related to the spatial density of pixels printed by lasers. Moiré patterns tend not to form with stochastic printing because stochastic printing does not employ rulings and screen angles. In stochastic printing, however, swath lines may generate undesirable banding artifacts.
To reduce the artifacts described above, techniques may be employed to break up and/or reduce the swath lines, thereby reducing banding. In general, the techniques provide for overlapping swaths and providing masking for one or both passes that print the overlapped region. In general, a mask is a set of logical ‘0’ values and logical ‘1’ values. The lasers in the array are individually modulated as a function of the image data and the mask. Depending upon whether a value is a logical ‘0’ value or a logical ‘1’ value, a laser in the imaging array is either enabled or deactivated. In this way, masking involves printing a fraction of the pixels.
Some masking techniques, while reducing some banding artifacts, may introduce new banding artifacts. In particular, each laser in the array, when printing, derives some benefit from the heat generated by its neighbor lasers. An end laser, which has only a single adjacent neighbor laser, derives less thermal benefit than other lasers and consequently may print lower density pixels. As a result, the line width of an end laser may be narrower than the line width of other lasers, which can lead to banding. Another potential source of banding is too much transfer of colorant caused by overlapping swaths. The banding artifacts are especially evident along the edges of the swath.